Many emergency vehicles are equipped with siren speakers for generating audio emergency warning signals. Such speakers are typically horn-loaded to enhance their efficiencies of sound generation.
One major disadvantage of conventional horn-loaded speakers is that they are typically very bulky, due to the horn length required to provide efficient coupling of acoustic energy to the external air. It is common to fold the horn structure of a horn-loaded speaker to reduce the overall length of the speaker. Nevertheless, even with a folded horn structure, a horn speaker still tends to be large and heavy.
The bulkiness of horn speakers makes their proper placement very difficult in many applications. This problem is especially serious for siren speakers designed for use on emergency vehicles. This is because siren speakers are typically add-on items to emergency vehicles, and it is often very difficult to find adequate space in an emergency vehicle to accommodate a horn-loaded siren speaker.
A significant progress in the reduction of the overall size of a horn-loaded siren speaker was made in a speaker disclosed in U.S. Pat. No. 4,893,343. This speaker has a conventional driver positioned to face the rearward direction of the speaker and a horn folded around the driver. The first section of the horn is formed in a horn plate which is disposed perpendicular to the longitudinal axis of the speaker. The driver is positioned in register with a central aperture in a horn plate which opens to the horn in the horn plate and forms the throat of the horn. The horn is bifurcated twice in the horn plate to form four passage ways. The speaker further includes a horn housing in which the driver is mounted. A dome-shaped cap is attached to the end of the driver assembly. The side walls of the housing and the surface of the dome cap define a horn mouth facing the forward direction of the speaker. The horn mouth is coupled to the passage ways in the horn plate for coupling the acoustic energy conducted therethrough to the external air.
The folding of the horn in the horn plate and around the driver results in a significant reduction of the length of the speaker. Nevertheless, even with the size reduction achieved with this construction, the length and weight of this speaker is still quite substantial. As a result, it is often hard to find an adequately sheltered location in an emergency vehicle for mounting the speaker. It is common to mount the speaker under the front bumper of an emergency vehicle. At such a location, the direct exposure to rain, wind, and dust significantly shortens the service life of the speaker.
Another common problem of horn-loaded siren speakers used on emergency vehicles is related to the cooling of the driver. A siren speaker used on an emergency vehicle is required to be capable of generating a very high sound level. During high-power operation, the heat generated in the voice coil and the magnet assembly of the driver has to be efficiently dissipated away. Accumulated heat in the driver can cause a loss of efficiency and in some cases may even cause the voice coil to be burned out or reduce the magnetic strength of the magnet assembly. Efficient heat dissipation, however, is difficult to achieve, especially in a design that tightly folds the horn structure about the driver to achieve a compact size. It is possible to use active means, such as a fan, to force air circulation around the magnet assembly. Such powered cooling, however, is undesirable due to the added power consumption, complexity, and cost of the speaker.